Among refractory products, a distinction is made between molten and cast products and sintered products.
Unlike sintered products, fused and cast products most often include a very abundant intergranular glassy phase which will fill the lattice of the crystallized grains. The problems encountered by sintered products and by fused and cast products in their respective applications, and the technical solutions adopted for solving them, are therefore generally different. Moreover, because of the significant differences between the making processes, a composition developed for making a fused and cast product cannot a priori be used as such for making a sintered product and vice versa.
Sintered products are obtained by mixing suitable raw materials and then shaping this mixture in the green state and baking the resulting green form at a sufficient temperature and during sufficient time in order to obtain sintering of this green form.
Sintered products depending on their chemical composition and their preparation method are intended for very diverse industries.
A sintered product adapted to a particular application will therefore not have a priori the properties required for it being used in another application where the temperature, corrosion or abrasion conditions are different.
For instance, U.S. Pat. No. 3,899,341 describes sintered products elaborated from zircon (50-90%) and from zirconia. The zirconia is partly stabilized in order to limit elastic deformations of the products, which lead to cracks. The products of U.S. Pat. No. 3,899,341 are however designed to be used in contact with molten steel. They are therefore not a priori suitable for being used in contact with molten glass.
Among the sintered products, the dense products elaborated from zircon (zirconium silicate: ZrO2.SiO2, or ZrSiO4) and optionally zirconia (free zirconium oxide ZrO2) may be used in applications where they are directly in contact with molten glass, in particular in the case of non-alkaline glasses.
EP 952 125 thus describes sintered products intended for glass furnaces and elaborated from zircon (5-40%) and from zirconia. These products further contain titanium, aluminium and yttrium oxides with which large blocks may be elaborated without any cracks. The SiO2 content of these products is less than 14%. Their ZrO2 content is larger than 82%.
WO 02/44102 describes isopipes used for making glass sheets. Isopipes include more than 95% by mass of zircon and exhibit insufficient creeping behavior, unless they also include between 0.2 and 0.4% of titanium oxide. In order to illustrate the state of the art, WO 02/44102 cites U.S. Pat. No. 5,124,287.
U.S. Pat. No. 5,124,287 describes compositions containing from 75 to 95% of zircon and of titanium oxide, intended to be in contact with molten glass. It indicates that the presence of titanium oxide is favorable for densifying the obtained products after sintering. In the final product, the zirconia should be non-stabilized and it is therefore preferable to use non-stabilized zirconia in the starting mixture. The use of stabilized zirconia, for example with stabilizers of zirconia such as yttrium or calcium oxides, is however not redhibitory, the heating of the mixture leading to destabilization of zirconia.
WO 2006/073841 describes refractory materials intended for the glass industry. These materials based on zircon, may include Y2O3. They always include at least 1% P2O5 or V2O5.
SU 1 020 404 describes a mixture intended for coating refractory products based on zircon.
The appearance of new glasses, such as alkali-free glasses, entails an increase in the melting and/or shaping temperatures for glass. Consequently, traditional materials based on zircon see their performances decrease. Indeed, traditional materials based on zircon tend to exudate when they are subject to temperatures above 1,550° C. Exudation is all the more significant since the temperature and the exposure time to these temperatures increase. This phenomenon is particularly damageable because it may then be accompanied by formation of defects in the glass, increased corrosion by the glass and by bubbling phenomena.
Therefore, there is a need for a product having very good resistance to exudation and which may be used in glass furnaces. The present invention is aimed at meeting this need.